The traditional view of the null subject as pro identified by Agr (the φ−features of I) cannot be maintained in a theory where Agr is uninterpretable. Two hypotheses are compared with regard to the predictions they make for Finnish null subject constructions: (A) Agr is interpretable in null-subject languages, and pro is therefore redundant; (B) Null subjects are specified but unpronounced pronouns which assign values to the uninterpretable features of Agr. Since Finnish observes the EPP and has an expletive pronoun, Hypothesis A predicts that null subjects should co-occur with expletives. The prediction is false, favouring B over A.A typology of null subjects is proposed: Null bound pronouns and null generic pronouns in partial null-subject languages, including Finnish, are D-less φPs, and so are null subjects in consistent null-subject languages with Agr, such as Spanish, Greek, etc. Null 1 st and 2 nd person subjects in Finnish are DPs which are deleted. Null pronouns in languages without Agr, such as Chinese or Japanese are the only true instances of pro, a minimally specified null noun.key words: null subject, empty category, phi-feature, expletive, pronoun,

The biotin-streptavidin interaction can be reversibly broken using water at elevated temperaturesThe biotin-streptavidin system is the strongest noncovalent biological interaction known, having a dissociation constant, K d , in the order of 4610 214 M. The strength and specificity of the interaction has led it to be one of the most widely used affinity pairs in molecular, immunological, and cellular assays. However, it has previously been impossible to re-use any streptavidin solid support, since the conditions needed to break the interaction with biotin has led to the denaturation of the streptavidin. Here, we show that a short incubation in nonionic aqueous solutions at temperatures above 707C can efficiently break the interaction without denaturing the streptavidin tetramer. Both biotin and the streptavidin remain active after dissociation and both molecules can therefore be re-used. The efficiency of the regeneration allowed solid supports with streptavidin to be used many times, here exemplified with the multiple re-use of streptavidin beads used for sample preparation prior to automated DNA sequencing. The results suggest that streptavidin regeneration can be introduced as an improvement in existing methods and assays based on the streptavidin system as well as emerging solid phase applications in fields, such as microfluidics and nanotechnology. IntroductionThe strong interaction between avidin and biotin was discovered as early as 1941 [1]. Avidin is a protein commonly purified from chicken egg white while biotin is a vitamin found in all cells. Streptavidin, a bacterial homologous protein to avidin, isolated from the actinobacterium Streptomyces avidinii, is more frequently used than avidin and is commercially available also in a number of engineered forms. The structure of the biotin-streptavidin complex has been described by several groups [2,3], showing a b-barrel structure of streptavidin binding biotin into its interior. The binding between avidin/streptavidin and biotin has long been regarded as the strongest, noncovalent, biological interaction known, having a dissociation constant, K d , in the order of 4610 214 M [4]. The bond forms very rapidly and is stable in wide ranges of pH and temperature [1,5].The strong interaction has led to a large number of research and diagnostic applications using avidin-biotin or streptavidin-biotin technology. The strength and reliability of the interaction underlie its importance in biotechnology, but the interaction is also a model for high-affinity receptor ligand binding. In most assays, streptavidin is coupled to a solid phase, such as a magnetic bead, a microtiter plate, or a biosensor chip, while biotin is coupled to the moiety of interest, often a nucleic acid, protein, or antibody. However, harsh conditions, such as formamide treatment combined with high temperatures, have been required to separate biotin from streptavidin, resulting in not only denatured streptavidin molecules [5] but also in limitations of downstream applications due to deterioration ...

No abstract